Induction heating apparatus such as induction furnaces or ladles for heating or melting metals operate on the principle of inducing eddy currents in an object (sometimes referred to as the load) to be heated. The eddy currents cause the load to act as its own heat source. Power is generated in the load by resistive heating caused by the eddy currents, according to the well-known P=I.sup.2 R heating principle. As used herein, "heating" is used broadly to encompass not only raising the temperature of a material without causing the material to change state, but also melting, wherein the temperature of a material is raised sufficiently to cause it to change state.
In a typical induction furnace, metal to be heated is contained in a crucible, and a generally helical induction coil surrounds the crucible. The induction coil is water cooled. The crucible is usually made of a ceramic refractory material. The eddy currents are induced in the load by passing a high-frequency alternating current through the induction coil to generate a time-varying magnetic field, or induction field. Depending upon the magnitude and frequency of the alternating current in the induction coil, and on other design considerations, the induction field can be used for melting, heating, and/or stirring a quantity of molten metal in the crucible. The induction field can also be used for heat treating workpieces, and for other procedures.
In virtually all coreless induction furnaces, the induction coil is constructed of several turns of heavy wall copper tubing shaped into the form of a helix. Alternating electrical current is conducted through the coil via termination tubes connected to the top and bottom turns of the coil. Heat generated in the coil turns is removed by water pumped through the copper tubing. Often, the same termination tubes are used to connect the coil to both a cooling water supply and a source of electrical current. As a practical matter, the termination tubes usually are located near each other at one end of the coil.
The coil is preferably made from one continuous length of copper tubing, or sections of tubing welded or brazed into one continuous length. One drawback of this method of construction is that is does not have great hoop strength. Another is that it is necessary to maintain large inventories of copper tubing for making coils, and to have machinery for winding the copper tubing (which is often of large diameter) into a helical shape. Welding or brazing lengths of copper tubing together to make a large enough coil present readily apparent disadvantages of their own.
The pitch of the helical winding, especially when large diameter tubing is used in high power furnaces, causes complications in mounting the coil in the furnace, which has flat top and bottom surfaces.
In conventional helically wound induction coils, current flows from the bottom turn to the top turn (or vice versa) and the returns vertically down (or up) via the termination tube to the bottom (or top) turn. While the magnetic field H.sub.c of the coil windings is concentrated inside the furnace in the direction of the axis of the coil, the magnetic field H.sub.t of the current in the termination tube is spread in the area around the tube in the plane of the coil turns. This stray magnetic field induces eddy currents in surrounding metal objects, causing them to become heated.